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Tagless, non-blocking data access library for Postgres.

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// Copyright (c) 2018-2021 by Rob Norris
// This software is licensed under the MIT License (MIT).
// For more information see LICENSE or https://opensource.org/licenses/MIT

package skunk.data

import cats.syntax.all._

/**
 * Cache based on a two-generation GC.
 * Taken from https://twitter.com/pchiusano/status/1260255494519865346
 */
sealed abstract case class SemispaceCache[K, V](gen0: Map[K, V], gen1: Map[K, V], max: Int) {

  assert(max >= 0)
  assert(gen0.size <= max)
  assert(gen1.size <= max)

  def insert(k: K, v: V): SemispaceCache[K, V] =
    if (max == 0)             this                                       // special case, can't insert!
    else if (gen0.size < max) SemispaceCache(gen0 + (k -> v), gen1, max) // room in gen0, done!
    else                      SemispaceCache(Map(k -> v), gen0, max)     // no room in gen0, slide it down

  def lookup(k: K): Option[(SemispaceCache[K, V], V)] =
    gen0.get(k).tupleLeft(this) orElse       // key is in gen0, done!
    gen1.get(k).map(v => (insert(k, v), v))  // key is in gen1, copy to gen0

  def containsKey(k: K): Boolean =
    gen0.contains(k) || gen1.contains(k)

}

object SemispaceCache {

  private def apply[K, V](gen0: Map[K, V], gen1: Map[K, V], max: Int): SemispaceCache[K, V] =
    new SemispaceCache[K, V](gen0, gen1, max) {}

  def empty[K, V](max: Int): SemispaceCache[K, V] =
    SemispaceCache[K, V](Map.empty, Map.empty, max max 0)

}